Typically, the optical systems used in the context of fabricating microelectronic devices such as semiconductor devices include a plurality of optical element units including optical elements, such as lenses and mirrors etc., arranged in the exposure light path of the optical system. Those optical elements usually cooperate in an exposure process to transfer an image of a pattern formed on a mask, reticle or the like onto a substrate such as a wafer. The optical elements are usually combined in one or more functionally distinct optical element groups. These distinct optical element groups may be held by distinct optical exposure units. In particular with mainly refractive systems, such optical exposure units are often built from a stack of optical element modules holding one or more optical elements. These optical element modules usually include an external generally ring shaped support device supporting one or more optical element holders each, in turn, holding an optical element.
Due to the ongoing miniaturization of semiconductor devices there is, however, a desire for enhanced resolution of the optical systems used for fabricating those semiconductor devices. This desire for enhanced resolution obviously pushes the desire for an increased numerical aperture (NA) and increased imaging accuracy of the optical system.
One approach to achieve enhanced resolution is to reduce the wavelength of the light used in the exposure process. In the recent years, approaches have been taken using light in the extreme ultraviolet (EUV) range, typically using wavelengths ranging from 5 nm to 20 nm, in most cases about 13 nm. In this EUV range it is not possible to use common refractive optics any more. This is due to the fact that, in this EUV range, the materials commonly used for refractive optical elements show a degree of absorption that is too high for obtaining high quality exposure results. Thus, in the EUV range, reflective systems including reflective elements such as mirrors or the like are used in the exposure process to transfer the image of the pattern formed on the mask onto the substrate, e.g. the wafer.
The transition to the use of high numerical aperture (e.g. NA>0.4 to 0.5) reflective systems in the EUV range leads to considerable challenges with respect to the design of the optical imaging arrangement.
The accuracy of the position of the image on the substrate is referred to as the line of sight (LoS) accuracy. The line of sight accuracy typically scales to approximately the inverse of the numerical aperture. Hence, the line of sight accuracy is a factor of 1.4 smaller for an optical imaging arrangement with a numerical aperture NA=0.45 than that of an optical imaging arrangement with a numerical aperture of NA=0.33. Typically, the line of sight accuracy ranges below 0.5 nm for a numerical aperture of NA=0.45. If double patterning is also to be allowed for in the exposure process, then the accuracy would typically have to be reduced by a further factor of 1.4. Hence, in this case, the line of sight accuracy would range even below 0.3 nm.
Among others, the above leads to very strict desired properties with respect to the relative position between the components participating in the exposure process as well as the deformation of the individual components. Furthermore, to reliably obtain high-quality semiconductor devices it is not only desirable to provide an optical system showing a high degree of imaging accuracy. It is also desirable to maintain such a high degree of accuracy throughout the entire exposure process and over the lifetime of the system. As a consequence, the optical imaging arrangement components, i.e. the mask, the optical elements and the wafer, for example, cooperating in the exposure process are supported in a well-defined manner in order to maintain a predetermined spatial relationship between the optical imaging arrangement components and to provide minimum undesired deformation as well to provide a high quality exposure process.
To maintain the predetermined spatial relationship between the optical imaging arrangement components throughout the entire exposure process, even under the influence of vibrations introduced, among others, via the ground structure supporting the arrangement and/or via internal sources of vibration disturbances, such as accelerated masses (e.g. moving components, turbulent fluid streams, etc.), as well as the under the influence of thermally induced position alterations, it is desirable to at least intermittently capture the spatial relationship between certain components of the optical imaging arrangement and to adjust the position of at least one of the components of the optical imaging arrangement as a function of the result of this capturing process. Similar applies to the deformation of at least some of these components of the optical imaging arrangement.
For example, deformable mirrors are known from US 2002/0048096 A1 (Melzer et al.) and U.S. Pat. No. 5,986,795 (Chapman et al.), the entire disclosure of each of which is incorporated herein by reference. Here, the deformable mirror arrangement is formed by a mirror body and a matrix of piezoelectric actuators arranged on the mirror rear side opposite to the optical surface of the mirror. The piezoelectric actuators typically act in a direction perpendicular to the optical surface between the mirror body and a counter structure (i.e. in push-pull configuration as shown e.g. in U.S. Pat. No. 5,986,795). As an alternative, in mirror intrinsic deformation systems, the piezoelectric actuators act exclusively on the mirror body introducing shear forces (typically parallel to the optical surface) to generate bending deformation of the mirror body (as shown e.g. in US 2002/0048096 A1). A similar configuration is also known from DE 10 2004 051 838 A1 (Möller et al.), the entire disclosure of which is incorporated herein by reference.
In these cases, attachment of the piezoelectric actuators to the mirror body is used for the long term imaging accuracy of the optical system. The most widely used attachment method in mirror intrinsic systems is gluing, which can take place at or near room temperature. However, the glue layer between the actuator and mirror body is subject to long term changes on a nanometer scale, e.g. due to stress relaxation, moisture absorption etc. This particularly applies under cyclic stress as it is typically the case in optical imaging systems.
In order to compensate for such long term changes, it is usually desirable to provide a sensor system capturing information representative of the deformation of the mirror and to provide a control system to take corrective actions. Such a sensor and control system adds greatly to the cost and complexity of the system. Moreover, in case of a failure of the sensor and control system, the deformation caused by dimensional changes of the adhesive layer can no longer be compensated for, i.e. the system cannot be operated further even when the deformation system is deactivated.
Other attachment methods, such as anodic bonding, frit bonding, thermosonic bonding, etc. all take place at elevated temperatures, typically around 200° C. This implies that in order to obtain a near stress-free assembly at room temperature, the mirror body and the piezoelectric actuators have to exhibit coefficients of thermal expansion (CTE), which closely match. This however poses certain issues, since there are very few glass materials and only a couple of specialized metallic materials, which are suitable for known mirror polishing processes and, at the same time, exhibit a coefficient of thermal expansion sufficiently closely matching the coefficient of thermal expansion of appropriate piezoelectric actuators.
A further issue involved with known intrinsic deformation systems are the parasitic stresses introduced into the mirror via the electrical connections to apply the electric field to each piezoelectric actuator. Multiple electrodes of the desired pattern are placed on the rear side of the mirror and are wired to their respective driving electronics to enable application of the electrical field. Most known wire attachment processes, such as soldering, ultrasonic bonding, gluing using conductive adhesives, etc. are not stress-free. This in turn causes deformation on the mirror surface (as the electrodes on the back side have to be placed directly under the optical footprint), which is further subject to drift and relaxation over lifetime.
This issue is common to all the aforementioned systems as well as to reflective systems with a deformation layer having front and rear electrodes forming part of a multilayer reflective optical surfaces as it is known for example from DE 10 2011 077 234 A1 (Dinger et al.), the entire disclosure of which is incorporated herein by reference. Similar applies to systems with a deformation layer having front and rear electrodes which are located between the optical element body and the reflective optical surface as it is known for example from DE 10 2011 081 603 A1 (Dinger et al.), the entire disclosure of which is incorporated herein by reference. Finally, this also applies to systems with a deformation sections having front and rear electrodes which are embedded in the optical element body as it is known for example from US 2015/0104745 A1 (Huang et al.), the entire disclosure of which is incorporated herein by reference.